This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from an application for Data Transmitter And Receiver Of A Spread Spectrum Communication System Using a Pilot Channel earlier filed in the Korean Industrial Property Office on 22 Nov. 1994 and assigned Ser. No. 30743/1994.
1. Technical Field
The present invention relates to a spread spectrum communication system, and more particularly to data transmitter and receiver of the spread spectrum communication system using a pilot channel.
2. Background Art
Conventionally, narrow band modulation systems (such as for example, amplitude modulation, frequency modulation and phase shift key modulation) have been used in the field of data communication. With such systems, demodulation at the receiver can be achieved with a relatively small amount of circuitry. Such systems, however, are weak due to multipath fading and narrow band noise.
By contrast, in spread spectrum communication systems, a data spectrum is spread by a pseudo-noise code (hereinafter xe2x80x9cPN codexe2x80x9d) at a transmitting side, while the pseudo noise code and the data are synchronized at a receiving side to that the adverse effects of multipath fading and narrow band noise can be reduced. Accordingly, spread spectrum communication systems have attracted increased attention as a promising technique for radio frequency transmission of binary data.
One example for such a spread spectrum communication system is disclosed in U.S. Pat. No. 5,400,359 entitled Spread Spectrum Communication System and An Apparatus For Communication Utilizing This System issued to Hikoso et al. on 21 Mar. 1995. In Hikoso et al. ""359, a pseudo noise code is generated and multiplied by data to generate a multiplied result which is then subjected to binary phase-shift key (BPSK) modulation, although other phase-shift key modulation such as, for example, differential phase-shift key modulation (DPSK) may also be used. The pseudo noise code is also subjected to BPSK modulation, delayed by at least one chip of the pseudo noise code, combined with a modulated signal, converted into a radio requency (RF) signal, and transmitted from an antenna. The transmitted spread spectrum signal is received at a receiving end where a complementary receiving method is provided. In essence, the spread spectrum communication involves the art of expanding the bandwidth of a signal, transmitting the expanded signal, and recovering the desired signal by remapping the received spread spectrum into the original information bandwidth. The purpose of spread spectrum techniques is to allow the system to deliver error-free information in a noisy signal environment.
In such a spread spectrum communication system however, since the spectrum of the information signal is spread by a PN code having a broader spectrum width, i order to correctly restore the information signal, it is necessary to synchronize the demodulation PN code which is generated at the receiving side with the modulation PN code which is generated at the transmitting side. Proper phase synchronization may be achieved when the received spread spectrum signal is accurately timed in both its spreading PN code pattern position and its rate of chip generation. The phase synchronization process is typically accomplished in two stages, i.e., an initial synchronization process for finding a synchronous phase and a process for tracking the detected phase. In these conventional spread spectrum receivers, however, initial synchronization and synchronization tracking are often achieved through costly and complex circuitry. Moreover, we have observed that it is difficult to adjust synchronization of the PN code at the receiving side, as the modulated PN code and not pure PN code is transmitted at the transmitting side. Consequently, the time required to establish initial synchronization has not effectively improved.
Accordingly, it is therefore an object of the present invention to provide a novel and improved spread spectrum communication system utilizing a pilot signal for establishing initial system synchronization.
It is another object of the present invention to provide an improved spread spectrum communication system utilizing a pilot signal for simplifying the synchronization process and minimizing the PN code acquisition time.
It is also an object of the present invention to provide an improved spread spectrum communication system including a transmitter and a receiver capable of utilizing a pilot signal for simplifying the synchronization process and minimizing the PN code acquisition time.
It is also an object of the preset invention to provide a improved spread spectrum communication system capable of providing the transmission and reception of a spread spectrum signal with low bit error rates.
It is a further object of the invention to provide an improved spread spectrum communication system capable of reducing peak-to-average power ratio (PAR) in a transmitter of a mobile communication system using at least two channels. 
To achieve the above objects of the present invention, the spread spectrum communication system includes a novel and improved transmitter and a complementary receiver capable of establishing a pilot channel for the transmission of pure rather than modulated PN codes for acquisition or tracking purposes.
To achieve the above objects of the present invention, the spread spectrum communication system is constructed to receive first and second input signals, to spread the first input signal with first and second spreading code signals to produce first and second spread signals, respectively, to spread the second input signal with the first and second spreading code signals to produce third and fourth spread signals, respectively, to produce a first output spread signal by subtracting the fourth spread signal from the first spread signal, and to produce a second output spread signal by adding the second spread signal to the third spread signal, so that the PAR could be reduced upon a radio transmission of the first and second output spread signals. 
The improved transmitter as constructed according to the present invention comprises a first Walsh orthogonal code generator for generating first and second Walsh orthogonal codes having respective Walsh orthogonal code systems; a Walsh orthogonal modulator for multiplying a predetermined pilot signal and data to be transmitted respectively by the first and second Walsh orthogonal codes and generating Walsh- orthogonal modulated pilot signal and data; a first PN code generator for generating first and second PN codes; a first band spreader for multiplying the Walsh-modulated pilot signal by the first and second PN codes, and generating I channel and Q channel band spreaded signals; a second band spreader for multiplying the Walsh-modulated data by the first and second PN codes, and generating I channel and Q channel band spreaded data; a finite impulse response filter for finite impulse response filtering the band spreaded pilot signals and data; a first converter for combining the I channel band spreaded pilot signal and data, and then converting into an I channel analog signal; a second converter for combining the Q channel band spreaded pilot signal and data and then converting into a Q channel analog signal; a lowpass filter for lowpass filtering the I channel and Q channel analog signals; an intermediate frequency mixer for receiving the lowpass filtered I channel and Q channel lowpass filtering signals and an intermediate frequency signal multiplying the I channel lowpass filtering signal by in phase component cosWIFt of the intermediate frequency signal, the Q channel low-pass filtered signal by a quadrature phase component sinWIFt of the intermediate frequency signal, and then combining the I channel and Q channel signals which have been mixed with the intermediate frequency; a carrier mixer for multiplying the output signal of the intermediate frequency mixer by a radio frequency signal cosWIFt; a bandpass filter for bandpass filtering the output signal of the carrier mixer; and a first amplifier for amplifying the bandpass filtered signal according to a predetermined amplification ratio for transmission via an antenna.
The complementary receiver as constructed according to the present invention comprises a second amplifier for amplifying a spread spectrum signal received via an antenna; a bandpass filter for bandpass filtering the output signal of the second amplifier; a first mixer for multiplying the output signals of the bandpass filter by the radio frequency signal cosWRFt, and converting into an intermediate frequency signal; a second mixer for multiplying the intermediate frequency signal by an in phase component cosWIFt and a quadrature phase component sinWIFt of the intermediate frequency, respectively, and then outputting I channel and Q channel signals from which the carrier frequency signal has been removed; a low-pass filter for low-pass filtering the I channel and Q channel signals, respectively; an analog-digital converter for converting the low-pass filtered I channel and Q channel signals into digital signals; a second PN code generator for generating first and second PN codes in response to a predetermined PN clock; an I channel despreader for multiplying the digital converted I channel output from the analog-digital converter by the first and second PN codes and then outputting a band despreaded I channel signal; a Q channel despreader for multiplying the digital converted Q channel output from the analog-digital converter by the first and second PN codes and then outputting a band despreaded Q channel signal; a PN code sync controller for Walsh- demodulating the band despreaded I channel and Q channel signals in response to the first Walsh code, detecting the PN code sync status of the Walsh-demodulated I channel and Q channel signals and then outputting a PN clock corresponding to the PN code sync status; a Walsh an orthogonal code generator for generating first and second Walsh codes having respective Walsh orthogonal code systems; a first Walsh demodulator for outputting first and second I channel signals which have been Walsh demodulated by the first and second Walsh codes; a second Walsh orthogonal code systems; for outputting first and second Q channel signals which have been Walsh- demodulated by the first and second Walsh orthogonal codes; an accumulator and dump circuit for accumulating and dumping the Walsh- demodulated first and second I channel signals and first and second Q channel signals; a combiner for receiving the first and second I channel signals and first and second Q channel signals output from the accumulator and dump circuit, and multiplying the first I channel signal by the second I channel signal to output a combined I channel signal and the first Q channel signal by the second Q channel signal to output a combined Q channel signal; and a data decider for obtaining a difference value between the I channel signal and Q channel signal output from the combiner and then deciding and outputting the data corresponding to the phase of the difference value.